The present invention relates to a method of manufacturing a circuit board capable of enhancing the heat dissipation effect by using a metallic base material as a base.
For example, a circuit board that mounts an electronic device evolving heat such as a semiconductor chip including a power FET uses a metallic base material in order to enhance the heat dissipation effect. The metallic base material includes copper or an aluminum raw material as a base. With a circuit board using a related art metallic base material as a base, a layer conductor (copper foil) is bonded to a metallic base material via a resin insulating layer and the unnecessary portion of the copper foil is dissolved and removed through etching to form a circuit pattern, the so-called subtractive method, is often used.
In this case, it is necessary to protect the metallic base material from a chemical liquid such as an etching liquid which is used to form a circuit pattern. This requires a step of covering the entire front and rear surfaces of the metallic base material with the resin insulating layer and a step of forming a protective resin insulating layer at the edges of the metallic base material.
Circuit boards are generally designed to enhance the heat dissipation effect by exposing a portion of the circuit board, preferably an end of the metallic base material formed in a rectangular shape and screwing the exposed portion onto a mounting part with a large thermal capacity such as a metallic mounting part. This requires a treatment step of exposing a portion of a metallic base material of a circuit board by shaving off a portion of the resin insulating layer after etching treatment.
As mentioned above, in case a surface cutting step of shaving off a portion of a resin insulating layer is employed after a step of forming a circuit pattern including the etching treatment, the corresponding treatment efficiency is extremely poor thus contributing to reduced mass productivity. Another problem is that chips of a resin insulating layer are deposited on a circuit pattern thus contaminating the same. Countermeasures against this problem are required.
As a method of manufacturing a circuit board without the need for protecting a metallic base material from a chemical liquid used for etching treatment has been proposed in JP-A-5-218642. According to the method of manufacturing a circuit board described in JP-A-5-218642, a first conductive layer is formed on a metallic base material via a first insulating layer and a second insulating layer with a second conductive layer formed thereon obtained in a separate step is stuck on the first conductive layer.
According to the description in JP-A-5-218642, a circuit pattern is formed on the second conductive layer through etching treatment on a copper foil in a separate step, so that there arises no special protection of the metallic base material as a base of the circuit board against the chemical liquid for etching treatment.
In the description in JP-A-5-218642, a method of sticking a copper foil punched into a pattern is used to form a first conductive layer on a metallic base material via a first insulating layer although a method may be used of forming a circuit pattern through etching treatment with a copper foil stuck on the entire surface. There is no description on practical measures of protecting a metallic base material during etching treatment in this example.